TOKYO—The fall of Mamoru Samuragochi, the charismatic composer who now admits he hired someone to create his works, may have begun in October with a magazine article by Takeo Noguchi, a music critic and conductor.

At the time, Mr. Samuragochi was at the peak of his fame in his home country as “Japan’s Beethoven,” and his work was set to accompany skater Daisuke Takahashi at the Sochi Olympics. But Mr. Noguchi thought he heard something out of tune in the Samuragochi persona, cultivated over more than a decade through interviews, books and a nationally broadcast documentary.

Mr. Samuragochi’s description of himself as the son of atomic-bomb survivors, completely deaf and wracked by migraines yet committed to his art, struck the critic as over the top. “I’ve never met him in person, but listening to his music and reading his books–which can be self-indulgently sentimental at times–I couldn’t help feeling something awkward,” he said in an interview.

Another trait seemed odd: Mr. Samuragochi’s music had strong similarities to works of masters like Bach, Mozart and Mahler. “Here’s a man who said he goes deep inside himself to pull out what he calls ‘authentic music,’ but then you hear all these references to the greats – it just didn’t match up,” Mr. Noguchi said.

The critic gave voice to his suspicions in an article titled “Is the ‘Deaf Genius Composer’ Mamoru Samuragochi Really Real?” and it caught the eye of Takashi Niigaki, who says that for 18 years, he ghost-wrote Mr. Samuragochi’s works.

This week, the drama came to its denouement: Mr. Niigaki went public with his story in a news conference and a tell-all article published in the magazine Shukan Bunshun. Mr. Niigaki told Shukan Bunshun that the suspicions expressed by the music critic were a factor in his decision to come forward.

Mr. Samuragochi admits he deceived the public by presenting another’s works as his own. Still in question is whether Mr. Samuragochi is deaf. Mr. Niigaki said that they had normal spoken conversations, while Mr. Samuragochi’s lawyer said he believed Mr. Samuragochi was truly deaf. Neither Mr. Niigaki nor the lawyer could be reached for comment Friday.

In the Shukan Bunshun piece, Mr. Samuragochi is portrayed as an ambitious man with a hunger for fame who persuaded Mr. Niigaki, then fresh out of music school, to provide musical scores for his projects. Mr. Niigaki said it was a chance to have his music heard and it didn’t matter that his name wasn’t credited.

The 43-year-old ghost composer said at the news conference that anxiety increased along with Mr. Samuragochi’s fame over the years. In March 2013, public broadcaster NHK featured Mr. Samuragochi’s story in its most prestigious programming slot, the documentary series “NHK Special.”

The film shows Mr. Samuragochi befriending a young girl whose mother was killed in the 2011 earthquake and tsunami. An NHK representative said the broadcaster had questioned those involved in producing the documentary and no one was aware that Mr. Samuragochi used a ghost writer.

“I felt I couldn’t go on when I submitted a piece for him last May, and I told him that in July,” Mr. Niigaki said at the news conference. “I asked again in December, but that didn’t work…and that led to this situation.”

Even as his world was about to collapse, Mr. Samuragochi kept up his image in public.

“I never doubted the authenticity of the music we performed,” said Kenji Igata, executive director of the Hiroshima Symphony Orchestra, who met Mr. Samuragochi when the orchestra performed Mr. Samuragochi’s “Symphony No. 1 ‘Hiroshima’ ” in late December.

Mr. Igata said the two exchanged greetings backstage through a sign-language translator. “He appeared to speak normally. It never crossed my mind that he might not be the composer of the music, or that he might not be deaf,” he said. “You know, he just looked like he does on television and that NHK documentary.”

The 50-year-old’s compelling and detailed back story helped create his mythic presence. In past interviews, he said he was taught the piano from the age of 4 and mastered Beethoven and Bach pieces by the age of 10.

In the Shukan Bunshun article, Mr. Niigaki said much of the profile was untrue. The episode of him being a piano prodigy appeared to be based on Mr. Niigaki’s childhood memories that he once recounted to Mr. Samuragochi. In fact, Mr. Samuragochi could only play introductory level piano and appeared to be ignorant of musical notation, Mr. Niigaki said at the news conference.

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